Why is Ken Mattingly replaced with Jack Swigert?

Ken Mattingly was replaced by Jack Swigert for the Apollo 13 mission due to concerns about exposure to measles. Mattingly had not had the vaccine and was at risk of contracting the disease after being in contact with a crew member's child who had measles. NASA made the decision to prioritize crew safety, leading to Swigert, who was already on standby, stepping in as the command module pilot.